1. Field of the Invention
The present invention relates to a piezoelectric element, a vibratory actuator including the piezoelectric element, and a drive unit including the vibratory actuator.
2. Description of the Prior Art
A vibratory actuator has been conventionally known which is for use in various electronic devices or other devices and includes a piezoelectric element (electromechanical transducer) (see, for example, Patent Document 1). This piezoelectric element is obtained by alternately stacking piezoelectric bodies and electrodes. For the vibratory actuator, the piezoelectric element is vibrated by applying voltages to the electrodes, thereby moving a movable body.
Patent Document 1: Japanese Translation of PCT International Application No. 2003-501988
A vibratory actuator needs two-phase power. A piezoelectric element of a vibratory actuator normally includes a piezoelectric body, and a feeding electrode and a common electrode which are independently provided to have two different phases, respectively. If the piezoelectric element is a single plane type piezoelectric element in which electrodes are formed on both surface of a single layer piezoelectric body and which has a thickness of several millimeters (mm), a voltage of several tens to several hundreds of volts has to be applied to sufficiently displace the piezoelectric element.
In recent years, the thickness of each piezoelectric body between electrodes is reduced by alternately stacking piezoelectric bodies and electrodes. Accordingly, even with the same magnitude of supply voltage, a voltage (field intensity) which is applied to each piezoelectric body per unit length is increased, thereby reducing a driving voltage.
However, for example, to realize 3 volt (V) driving using a lithium ion power supply, which has been used as a power source for recent cellular phones, the thickness of the piezoelectric body has to be reduced to several tens of micrometers (μm) or less. However, with such a small thickness of the piezoelectric body, a piezoelectric constant is reduced and the ratio of electrodes to the piezoelectric body is increased, thus resulting in reduction in efficiency.
In view of the efficiency and the number of devices for use, as a piezoelectric element driving method, a half bridge driving scheme or a single driving scheme is used in general. Either of those schemes allows only application of a voltage corresponding to a difference between a ground level and a power supply level to the piezoelectric element.
Using a full bridge driving scheme or a push-pull driving scheme, a reverse voltage of the power supply voltage can be applied to the piezoelectric element. Thus, even when a voltage of the same magnitude is applied thereto, twice the magnitude of voltage can be applied.
However, in a known vibratory actuator having a common electrode, a power supply circuit such as a full bridge circuit, a push-pull circuit and the like which is capable of applying a reverse voltage can not be used.
As a possible solution to this problem, a common electrode is divided. However, if each common electrode is merely divided, eight connection points with external electrodes, wires and the like are needed at an outer surface (for example, a surrounding surface) of a piezoelectric element. This causes deterioration of the properties of the piezoelectric element due to the weight of wires and the like.
Furthermore, with divided common electrodes provided, when a connecting electrode for connecting the external electrodes with one another is formed at the outer surface (for example, the surrounding surface) of the piezoelectric element, the number of the connection points can be reduced to four. However, in this case, an electrode has to be formed at each edge portion of the piezoelectric element (by so-called edge connection) and, when the piezoelectric element is brought in contact with peripheral equipment thereof, the electrode provided at the edge portion comes off or like problems arise. This results in reduction in reliability thereof.
In view of the above-described problems, the present invention has been devised and it is therefore an object of the present invention to allow full bridge driving in a piezoelectric element, a vibratory actuator and a driving unit while achieving improved reliability thereof.